This guide gives you a simple, reliable way to size the series resistor for LED circuits. Use the calculator below, then follow the step-by-step explanation to understand what's happening and how to design for safety and long life.

LED Resistor Calculator

Interactive LED Resistor Calculator

Volts
Volts per LED (typical: Red~2.0, White~3.2)
mA per branch (10–20 for small LEDs)
1 = single LED; 2/3+ for series strings
Use 1 resistor per branch

How the Calculation Works

For each branch of a series string:

  1. Total LED drop: Vf_total = Vf × (LEDs in series)
  2. Resistor drop: Vr = Vs − Vf_total
  3. Resistor value: R = Vr / If
  4. Resistor power: P = Vr × If

Use a resistor power rating with margin (at least 2× the calculated P). For parallel setups, size the resistor per branch. Total supply current is branches × If.

Worked Examples

Scenario Inputs Result Notes
Single red LED @ 20 mA from 12 V Vs=12, Vf=2.0, series=1 R=500 Ω → E12≈510 Ω; P=0.20 W Choose ≥0.5 W for comfort
White LED @ 15 mA from 9 V Vs=9, Vf=3.2, series=1 R≈387 Ω → E12≈390 Ω; P≈0.087 W 1/4 W is sufficient
3× red in series @ 20 mA from 12 V Vs=12, Vf=2.0, series=3 R=300 Ω; P=0.12 W Round to 330 Ω if you prefer cooler operation

Design Tips & E12 Rounding

  • E12 rounding: Prefer rounding up if you want lower current and longer life; nearest is a balanced default.
  • Series first: When Vs allows, use more LEDs in series to reduce wasted heat in the resistor.
  • Per-branch resistors: Never share one resistor across parallel LEDs; tolerances cause current hogging.
  • Automotive 12 V: Expect 11–14.4 V. Add margin or use a constant-current driver for consistency.
  • PWM dimming: Keep the series resistor; use PWM with a transistor/MOSFET for brightness control.
See also:

FAQs

Why is my LED dimmer than expected?

Likely the resistor value is high (e.g., rounded up), supply voltage is lower than assumed, or Vf is higher than typical. Measure Vs and Vf, then recompute.

Can I place the resistor on the cathode side?

Yes. Series placement is what matters; anode or cathode side behaves the same.

How much wattage margin should I choose?

At least 2× the computed P. Go higher (0.5–1 W) in hot environments or automotive systems.

When should I switch to a constant-current driver?

Use a driver for high-power LEDs, long strings, or when supply voltage varies significantly. It improves accuracy, efficiency, and lifetime.